In order to improve the performance of MOSFET devices the well known polysilicon is being replaced as gate electrode material by metals. Such metal gates do not suffer from the shortcomings of a semiconducting gate electrode such as gate depletion, dopant diffusion or medium range resistance. However, obtaining the desired work function for both nMOS and pMOS devices in view of controlling the threshold voltage is not as straightforward as in the case of polysilicon gates, as the work function of polysilicon can be tuned by doping the polysilicon with a well-defined amount of an appropriate dopant, e.g., boron for pMOS devices, thereby forming a high work function gate electrode, and phosphorus for nMOS devices, thereby forming a low work function gate electrode.
Initially, midgap metals were introduced to form metal gates. Midgap metals are materials having a Fermi level near the midgap position between the valence and conduction band of the semiconducting substrate on which the MOSFET is formed. An example of such midgap metal is CoSi2 having a work function of about 4.65 eV. This metal gate material is formed by a full silicidation (FUSI) of a polysilicon gate electrode whereby essentially all the polysilicon is converted into a silicide during reaction with Co. Although symmetric threshold voltages for nMOS and pMOS devices can be obtained using such a midgap metal, these threshold voltages are too high compared with the supply voltage at which these NMOS and pMOS are to operate.
In order to obtain the desired, low, threshold voltage for nMOS and pMOS devices a dual metal gate approach is currently being pursued. Here, the material of the gate electrode of the NMOS device is a metal having a low work function, ideally about 4 to 4.2 eV, while the material of the pMOS gate electrode is another metal having a high work function, ideally about 5 to 5.2 ev. Finding an appropriate low-work function metal for use as NMOS gate electrode is of particular importance, as such metal reacts rapidly with the oxygen present in the underlying dielectric, e.g., silicon-oxide or high-k dielectrics.
Tuning of the work function of a gate electrode metal by alloying it with other metals, in order to match the work function requirements for an nMOS or pMOS gate electrode, is known.
V. Mistra et al. discloses in United States Publication No. US-2003/0160227-A1 the alloying of a first metal having a higher work function with a second metal B having a lower work function. Depending on the ratio, the resulting alloy AxB1-x will have a work function ranging from the work function of the first metal A to the work function of the second metal B. The lowest work function obtainable is determined by the work function of the second metal B, which is selected from the group of Mn, Mg, V, Ti, Cr, Y, Zr, Ta, La, Gd, Sm, Pr, Nb, Al, Hf and alloys thereof. Mistra et al. does not teach the formation of a fully silicided gate electrode using such binary alloys.
J. Liu et al. discloses in “Dual-Work-Function Metal Gates by Full Silicidation of Poly-Si With Co—Ni Bi-layers”, IEEE Electron Device Letters 2005, the formation of dual work function metal gates fabricated by full silicidation (FUSI) of a Co—Ni bi-layer and a comparison of these alloyed metal gates with single-metal FUSI systems of CoSi2 and NiSi. The alloy of CoxNi1-xSi allows combining the low work function of NiSi with the thermal stability of CoSi.
Yu et al. discloses in “Low Work function Fully Silicided Gate on SiO2/Si and La AlO3/GOI n-MOSFETS”, Proceedings of 62nd Device Research Conference (DRC), pp. 21, Notre Dame, Ind., June, 2004, the combination of the low resistive NiSi, having a high work function of about 4.9 eV, with Hf or TiSi to obtain a low work function of 4.2 eV and 4.3 eV respectively.